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Solid-State Disk with RAC

Oracle Real Application Clusters(RAC) is a virtualized memory system that depends on a high speed interconnect to provide a single memory image across multiple host machines. This concept is illustrated in Figure 3.1. 

Performance in a RAC environment is generally maximized when a majority of data is cached within the virtualized memory area. Once a database is fully cached, the performance of the system then becomes dependent upon the speed of the memory-to-memory transfers via the interconnect. As a result, the overall performance of the entire system will be dependent on the speed and latency of the cluster interconnect protocol.

When the system is not fully cached, performance becomes codependent on the underlying disk I/O subsystem and the interconnect with a convoy effect. The convoy effect is a term used to describe how a set of dependent objects such as ships, cars, or computers becomes limited by the speed of the slowest member. In most computer systems where there is an interconnect and disks, the disks are the slowest performers.  Therefore, the speed at which an I/O subsystem can service requests depends ultimately on the latency of the systems disks, even when masked with memory caches.

The I/O subsystem consists of device types which map to drivers.  In turn, the drivers send the requests for I/O to a host bus adapter (HBA). The HBA converts the request to the proper format for the protocol being used, such as fiber, copper, or other protocols.  The request is then transferred over that protocol to the storage subsystem I/O bus to the storage controller, depending on whether the request can be satisfied via a cache read or must be read from disk the controller either reads from cache or disk.  A typical subsystem is shown in Figure 3.2. 

Of course the disk I/O subsystem also allows virtualization of the true underlying disk structures as shown in Figure 3.3. All of this I/O subsystem and disk activity can exact an extreme penalty on Oracle and Oracle RAC performance if any piece of the underlying structure is improperly configured.

Possible configuration issues include the SCSI interface, the proper use of disk managers, proper application of striping technology, proper stripe width and depths and the list goes on and on. Many times the system administrator and DBA throw up their hands and just accept whatever the vendor does to setup the disks and I/O subsystem. This often leads to sub-optimal performance.

An example of a disk array parameter that directly impacts performance is disk stripe width. For the purpose of this text, disk stripe width defined to mean the amount of a stripe in a RAID configuration that resides on one disk. In early systems, the advice of many experts was to stripe shallow and wide.  For example, one might use an 8k stripe width across 32 drives. This was based on rapidly retrieving a single large file. However, Oracle usually retrieves small reads such as single data values after index lookups. It does hundreds, if not thousands, of these in a single second in large systems. This means Oracle disk arrays must be tuned for concurrency. In Oracle systems, a stripe width that is too narrow is guaranteed to generate I/O contention. Oracle recommends a stripe width of between 128K (non-data) to 1 megabyte (data).

With any physical device that depends on the mechanical motion of actuator arms and spinning disks, there is also the issue of disk arm positional latency and disk rotational latency.  They severely limit the effective speed of I/O for physical disks.  Generally, non-linear I/O rates are only in the tens to hundreds of megabits per second. Many disk array manufacturers are getting around this through the implementation of huge memory caches, so not only do users get to pay for the disks, they get to pay for the memory caches as well. With disks running anywhere from $500-$3000 US, depending on the vendor from which they are acquired and what firmware has been loaded onto them, paying for both seems a bit redundant. In an SSD environment, the disks are there strictly as a backing store, since no large I/O rate is needed for this, there is no need to buy five or ten disks to get the I/O needed.  Just one can cover several units; although for redundancy, the backing store is usually mirrored.

Solid-state disk (SSD) technology is replacing ancient spinning platters of magnetic coated media with an array of super fast solid-state RAM.  Today’s SSD devices achieve tertiary storage with software mechanisms that write the RAM frames to a back end disk on the device.

With the cost of SSD at only $1k/gig, many Oracle systems administrators are exploring how to leverage this powerful performance tool for their environment.  Smaller databases can now run fully cached with SSD, yet there is a debate about the proper use of SSD in an Oracle environment.  Should it be used for redo? How about undo segments? Maybe temp? This debate becomes even more muddled with real application clusters (RAC) thrown into the picture.

With a standard Oracle database, the SSD array, when not used for the entire database, is best used for the files where the most I/O contention exists. For example, in an online transaction system indexes, redo, or temp may be best placed on SSD, while in a decision support or warehouse, temp and data files may be the best choice.

As this study has determined, RAC files placed on SSD may differ completely from those that might be considered in a non-RAC environment.


The above book excerpt is from:

Oracle RAC & Tuning with Solid State Disk

Expert Secrets for High Performance Clustered Grid Computing

ISBN 0-9761573-5-7

Donald K. Burleson & Mike Ault

http://www.rampant-books.com/book_2005_2_rac_ssd_tuning.htm

  
 

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